It is well known that the cAMP plays a major role in regulating cardiac function. Difference in adenylyl cyclase (AC) isoforms is a potential mechanism by which the cAMP signal, a common second messenger signal, can be regulated in a tissue-specific manner. However, the physiological significance of expressing multiple AC isoforms in a tissue and how each specific isoform regulates the cAMP signal remains largely unknown. In a genetically engineered mouse model in which the expression of the type 5 adenylyl cyclase (AC-5) is knocked out (AC-5 KO), we identified the loss of autonomic regulation and Ca2+-mediated inhibition of cardiac function. Accordingly, our current hypotheses are 1) Cardiac AC-5 plays a key role in regulating b-AR signaling and in modulating these effects by parasympathetic stimulation and intracellular Ca2+. This sets the stage for differential effects of b-AIR stimulation on cardiac function in vivo, where parasympathetic regulation is intact, and in vitro, where it is absent, 2) The ablation of AC-5 protects against the development of cardiac hypertrophy, apoptosis, and heart failure in response to pressure-overload and catecholamine-induced stresses, and 3) Deleting AC-5 will result in novel changes in gene and protein expression. These diverse approaches working in concert will yield new, important information on the mechanisms and molecular determinants of AC regulation under normal condition, and under the development of hypertrophy and heart failure. Thus the results of the current investigation will lead to new basic knowledge, which should also be valuable in providing new therapeutic direction to prevent or treat cardiac failure. More broadly, the multiple approach applied in transgenic mouse models, will provide an important experimental model system for understanding integrative physiology of AC regulation and testing potentially novel gene therapies for cardiovascular diseases. For example, the preliminary date in this project, if corroborated, suggest that inhibition of AC-5 could be a new pharmacological target, because of its beneficial role affording protection to the hypertrophied and failing heart.